How do you work out the ABV of a blend?
By volume. Alcohol is conserved when you pour one liquid into another — nothing is created or destroyed — so the blend holds exactly the alcohol that went in, spread through the new total volume. Ten liters of 4% into ten liters of 8% gives twenty liters of 6%.
ABV = (V₁ × ABV₁ + V₂ × ABV₂) ÷ (V₁ + V₂) · (10 × 4 + 10 × 8) ÷ 20 = 6%
The gravity blends the same way, for the same reason: mass and volume both add up, so the blend's specific gravity is the volume-weighted average of the two. What matters is that these are two separate calculations. One tracks alcohol, the other tracks sugar, and the answer to one tells you nothing about the other.
Why the blend's gravity won't tell you its ABV
This is where blending calculators — and forum answers — go wrong. ABV normally comes from a fermentation history: the distance between the original gravity and the final one. A blend has no such history. It has a gravity, and it has an alcohol content, and they are independent. Here are three blends that all finish at exactly the same gravity:
| 10 L + 10 L of… | Gravities | Blend reads | Blend ABV |
|---|---|---|---|
| A mild and a bitter | 1.016 + 1.024 | 1.020 | 4.4% |
| A pale ale and a stout | 1.012 + 1.028 | 1.020 | 6.5% |
| Two barleywines | 1.018 + 1.022 | 1.020 | 9.5% |
Equal volumes, so every blend lands on 1.020. Computed by this page's engine.
Same hydrometer reading, 4.4% to 9.5% alcohol — more than double, across blends a hydrometer cannot tell apart. If you took that last blend's 1.020 and ran it through the standard ABV formula against a plausible 1.060 original gravity, you'd get 5.3% — off by 4.2 points of alcohol. The formula isn't wrong; it's being asked a question about a fermentation that never happened.
You track it, rather than measure it. Work out each batch's ABV from its own OG and FG — that's what the ABV calculator is for — and then let this page do the weighted average. Once the liquids are mixed, no hydrometer reading will recover it. The only instrument that measures alcohol directly is a distillation or a vinometer, and neither belongs in a homebrew kitchen.
How much of each to hit a target ABV?
This is Pearson's square, the standard blending method in wine and spirits work, and it's just the weighted average solved backwards. To make 20 L at a target from a 9.4% batch and a 4.2% one:
| Target | 9.4% batch | 4.2% batch | Share strong |
|---|---|---|---|
| 5% | 3.1 L | 16.9 L | 15% |
| 5.5% | 5 L | 15 L | 25% |
| 6% | 6.9 L | 13.1 L | 35% |
| 6.5% | 8.8 L | 11.2 L | 44% |
| 7% | 10.8 L | 9.2 L | 54% |
| 7.5% | 12.7 L | 7.3 L | 63% |
| 8% | 14.6 L | 5.4 L | 73% |
20 L total. Nothing outside 4.2–9.4% is reachable — blending cannot go past its own components. Computed by this page's engine.
That last line is the guard rail worth remembering: a blend can only land between its two parts. If you want a beer stronger than everything you have, blending is the wrong tool — you need a stronger batch, or a spirit.
Fortifying a mead or cider with spirit
Adding a bought spirit is the same arithmetic with a very strong, very dry component. It's how port and other fortified wines are made: alcohol above roughly 18% stops the yeast, so fortifying an unfinished batch leaves the remaining sugar unfermented and sweet. For 19 L of 12% mead and a 40% spirit:
| Target | Spirit to add | Ends up | Hydrometer reads | Mead now |
|---|---|---|---|---|
| 14% | 1.46 L | 20.5 L | 1.015 | 93% strength |
| 15% | 2.28 L | 21.3 L | 1.012 | 89% strength |
| 16% | 3.17 L | 22.2 L | 1.010 | 86% strength |
| 18% | 5.18 L | 24.2 L | 1.005 | 79% strength |
| 20% | 7.6 L | 26.6 L | 1.000 | 71% strength |
From 19 L at 12% and 1.020, spirit at 40% and SG 0.95. "Mead now" is how concentrated the original is after dilution. Computed by this page's engine.
Take that mead to 15% and it needs 2.28 L of spirit — and the reading falls from 1.020 to 1.012. Nothing fermented. Ethanol is lighter than water — a 40% spirit has a specific gravity of about 0.95 — so you've both diluted the sugar and added something buoyant. Read that drop as attenuation and you'll conclude a finished batch restarted.
The last column is the one people forget. Everything in the original is now at 89% of its old concentration — the sugar, yes, but also the acid, the tannin and the aroma you spent a year building. Fortifying is not a free operation; it is a dilution that happens to raise the alcohol.
The one blend that can hurt you
Every calculation on this page is about strength. The risk isn't. If you blend a batch that still holds unfermented sugar into one that still holds live yeast, you have not made a blend — you have made a fresh fermentation, and if it happens in sealed bottles it makes pressure.
- Blending a sweet batch into a dry one restarts it. The dry batch's yeast is alive and hungry; the sweet batch is lunch. A stable gravity in each carboy separately tells you nothing about the blend.
- Confirm the blend is stable, not the parts. Blend, leave it somewhere warm for two weeks, and take gravity readings days apart. Package when the blend stops moving.
- Stabilizing is not sterilizing. Potassium sorbate stops yeast reproducing; it does not kill what's there and it will not stop a fermentation that's already going. With sulfite, in a batch that has genuinely finished, it holds a sweet wine, mead or cider — it is not a licence to bottle a sweet blend with live yeast.
- Fortifying to 18%+ is the exception, and it's why port is made that way: at that strength the yeast is finished, which is what leaves the sugar in place.
This is the one place where the arithmetic being right doesn't make you safe. Getting a blend to 15% is a calculation; knowing whether it will keep fermenting is an observation, and it takes two weeks and a hydrometer.
What blending fixes — and what it only spreads
Blending is a legitimate, professional technique: it's how large breweries hit the same gravity every batch, and how a too-hot barleywine becomes drinkable. Used forward — to balance, to soften, to hit a number — it works, and the math above is all you need.
Used as a rescue, it has one hard limit that the pages selling you blending charts don't mention. Blending cannot subtract a fault. It can only spread it. Pour 10 L of infected, oxidized or phenolic beer into 10 L of good beer and you do not get 10 L of good beer back. You get 20 L in which the fault sits at 50% of its old concentration — quieter, but present in every glass, and now in twice as much beer.
That trade is sometimes worth making. A fault sitting just above the threshold where you notice it can drop below it, and a batch you'd have poured away becomes drinkable. But it's a judgment about a threshold, not a repair — and if the fault is a live infection, it is still alive in the blend, and it will keep going. Taste a measured sample blend in a glass before you commit two carboys to it.